Internal combustion engines generate exhaust as a by-product of fuel combustion within the engines. Engine exhaust contains, among other things, unburnt fuel, particulate matter such as soot, and harmful gases such as carbon monoxide or nitrous oxide. To comply with regulatory emissions control requirements, engine exhaust must be cleaned before discharge into the atmosphere.
Engines typically include after-treatment devices that remove or reduce harmful gases and particulate matter in the exhaust. For example, a diesel engine can be equipped with a filter assembly consisting of a diesel oxidation catalyst (DOC) that promotes oxidation of unburnt fuel, carbon monoxide and/or nitrous oxide, and a diesel particulate filter (DPF) that traps particulate matter. Over time, the increasing volume of trapped soot impedes the flow of exhaust through the DPF and degrades engine performance. One commonly used technique for in-situ cleaning or regeneration of a DPF involves raising the temperature of the DPF above a combustion or oxidation threshold of the soot particles accumulated on the DPF. In most cases, this is achieved by heating the exhaust before it enters the DPF. When the hot exhaust interacts with the soot particles, they oxidize.
The temperature of exhaust flowing through a DPF can be raised in many ways. For example, engine operating parameters such as the fuel-air mixture composition or engine load can be varied to produce exhaust having a higher temperature. Alternatively, fuel can be injected directly into the exhaust and oxidized in the presence of the DOC at a location upstream of the DPF to raise the temperature of the exhaust. In this arrangement, the DOC, together with the fuel injectors or dosers, acts as an exhaust heater.
A DOC typically becomes active, however, only above a threshold temperature, known as the DOC light-off temperature. When a temperature of the exhaust exceeds the DOC light-off temperature, the DOC promotes oxidation of fuel injected in the exhaust via an exothermic reaction. At low engine loads, however, the temperature of the exhaust may remain below the DOC light-off temperature. In such cases, to activate the DOC, it may be necessary to pre-heat the exhaust before it interacts with the DOC.
One attempt to address the problems described above is disclosed in U.S. Pat. No. 7,406,822 of Funke et al. that issued on Aug. 5, 2008 (“the '822 patent”). In particular, the '822 patent discloses a particulate trap regeneration system, which includes a particulate trap used to remove one or more types of particulate matter from an exhaust flow of an engine, and an electric heating element or burner located upstream of the particulate trap. The '822 patent further discloses that the electric heater or burner can be used either to heat the exhaust conduit or the exhaust gases before they pass through the particulate trap.
Although the system of the '822 patent, discloses a burner to heat exhaust gases, the burner may heat the exhaust gases non-uniformly. Because the burner of the '822 patent applies heat locally, exhaust gases adjacent to the burner flame may be heated more than exhaust gases remote from the flame. Such non-uniform heating of the exhaust gases may result in incomplete regeneration of the DPF. Moreover, the temperature gradients may induce thermal stresses in the DPF and/or an associated DOC, causing them to break or be damaged.
The heater tube of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.